Disc refiners are used in the papermaking industry to prepare paper pulp fibers for the forming of paper on a papermaking machine.
Paper stock containing two to five percent dry weight fibers is fed between closely opposed rotating discs within the refiner. The refiner discs perform an abrading operation on the paper fibers as they transit radially between the opposed moving and non-moving refiner discs. The purpose of a disc refiner is to abrade the individual wood pulp fibers. A necessary corollary to that action is that a certain amount of abrasive wear of the refiner plates must occur.
Processing of fibers in a low consistency refiner may be performed on both chemically and mechanically refined pulps and in particular may be used sequentially with a high consistency refiner to further process the fibers after they have been separated in the high consistency disk refiner. In operation, a low consistency disc refiner is generally considered to exert a type of abrasive action upon individual fibers in the pulp mass so that the outermost layers of the individual cigar-shaped fibers are frayed. This fraying of the fibers, which is considered to increase the freeness of the fibers, facilitates the bonding of the fibers when they are made into paper.
Paper fibers are relatively slender, tube-like structural components made up of a number of concentric layers. Each of these layers (called "lamellae") consists of finer structural components (called "fibrils") which are helically wound and bound to one another to form the cylindrical lamellae. The lamellae are in turn bound to each other, thus forming a composite which, in accordance with the laws of mechanics, has distinct bending and torsional rigidity characteristics. A relatively hard outer sheath (called the "primary wall") encases the lamellae. The primary wall is often partially removed during the pulping process. The raw fibers are relatively stiff and have relatively low surface area when the primary wall is intact, and thus exhibit poor bond formation and limited strength in the paper formed with raw fibers.
It is generally accepted that it is the purpose of a pulp stock refiner, which is essentially a milling device, to partially remove the primary wall and break the bonds between the fibrils of the outer layers to yield a frayed surface, thereby increasing the surface area of the fiber multi-fold.
Disc refiners typically consist of a pattern of raised bars interspaced with grooves. Paper fibers contained in a water stock are caused to flow between opposed refiner discs which are rotating with respect to each other. As the stock flows radially outwardly across the refiner plates, the fibers are forced to flow over the bars. The milling action is thought to take place between the closely spaced bars on opposed discs. It is known that sharp bar edges promote fiber stapling and fibrillation due to fiber-to-fiber action. To achieve this, an advantageous method of fabricating bars which wear sharp has been utilized in the construction of refiner plates such as disclosed in U.S. Pat. No. 5,165,592 to Wasikowski. It is also known that dull bar edges result in fiber cutting by fiber-to-bar action.
Thus the material from which refiner disks are made should have high wear resistance. Wear resistance is typically associated with hard brittle materials, for example metal carbides. Refiner plates are subject to a corrosive environment. The pulp fibers are often contained in a stock which is acidic or basic as a result of the chemical processes used to free the wood fibers from the lignin which binds the fibers together in unprocessed wood. In addition to abrasive wear and corrosion, refiner plates can be subjected to impact loading as a result of opposed plates coming into contact or a foreign object impacting the plates. Failure of the plate due to lack of toughness can not only result in the destruction of the disk refiner but can damage downstream equipment.
A conflict is created by the need for both toughness and wear resistance in refiner plate materials which is further complicated by the need for good corrosion resistance. Low carbon stainless steel materials are normally used in refiner plate applications that require toughness. The properties of these stainless steel alloys are greatly influenced by carbon. Very low carbon levels are required to develop the excellent toughness and corrosion resistance that make stainless steels effective as refiner plate materials. Low carbon content, however, also translates into low hardness levels and poor resistance to abrasive wear. It has been a constant dilemma trying to improve these properties without greatly affecting the material's ability to resist breakage.